The present invention relates to a biaxially oriented polypropylene film and its use as wrap-around label.
Label films comprise an extensive and technically complex area. Different labelling techniques are distinguished, which are basically different regarding the process conditions and which inevitably make different demands of the label materials. It is common to all labelling processes that optically pleasing labelled containers have to result.
Very different techniques for applying the label are used with the labelling methods. It is distinguished between self-adhesive labels, wrap-around labels, shrink labels, in-mould labels, patch labelling etc. The use of a film of synthetic thermoplastics as label is possible in all these different labelling methods.
All in-mould labelling methods have in common that the label takes part in the actual forming method of the container and is applied in the meantime. Different forming methods are hereby used, like for example injection moulding methods, blow moulding methods, deep drawing methods.
In the injection moulding method, individual labels are taken from a stack or cut from a roll and are inserted into the injection moulding form. The form is thereby designed in such a manner that the melt flow is injected behind the label and that the front side of the film abuts the wall of the injection moulding form. The hot melt combines with the label during the injection. The tool opens after the injection, the injection moulding article with the label is ejected and cools down. As a result, the label has to adhere without creases and optically in a faultless manner on the container.
With blow moulding, a direct in-mould labelling is also possible. With this method, a melt tube is extruded vertically downwards through a ring nozzle. A vertically-divided forming tool closes and surrounds the tube, which is thereby squeezed together at the lower end. A blow mandrel is introduced at the upper end, through which the opening of the moulded article is formed. Air is introduced to the warm melt hose via the blow mandrel, so that it extends and abuts the inner walls of the moulding tool. Hereby, the label has to join with the viscous plastics of the melt hose. Subsequently, the mould is opened and the excess at the formed opening is cut. The formed and labelled container is ejected and cools down.
With deep drawing, non-oriented thick plastic plates, mostly cast PP or PS (polystyrene) with a thickness of about 200-750 μm are heated and drawn or pressed into a corresponding forming tool by means of a vacuum or stamping tools. Hereby, the individual label is also inserted into the form and joins with the actual container during the forming process. Considerably lower temperatures are used, so that the adhesion of the label at the container can be a critical factor.
Basically, films of synthetic thermoplastics can be used for the labelling of the containers during the forming with all these in-mould methods. For this, the films have to have a chosen property profile, so as to ensure that the label film and the formed mould body abut smoothly and without blisters and join with one another. The adhesion of the label to the container is often inadequate. Furthermore, air bubbles occur between the label and the container, which impair the appearance of the labelled container, and also the adhesion. With in-mould labelling, the speed of the process is essentially determined by the time which is necessary for the forming of the container. The corresponding cycle times, with which the labels are possibly destacked and handled in these methods, are comparatively moderate.
In the prior art, various films are described which are optimised in view of their use as in-mould label. These films often comprise a rough inner surface, that is facing the container, so as to avoid the air bubbles between the container and the label. The outer surface is however optimised in such a manlier that no boundary can be seen between the applied label and the container, for which reason the in-mould labels have glossy outer surfaces. The destacking of such films with in-mould labelling is unproblematic.
In addition to the in-mould labelling, the wrap-around labelling for labelling of non-conical containers and bottles is also very common due to cost reasons. Here, paper is also increasingly substituted by thermoplastic films.
With the wrap-around labelling, a label section corresponding to the print rapport is cut to length, this label section is wrapped around the container, so that the facing edge regions overlap. The edges are glued e.g. with a hot melt adhesive at the overlap, whereby the outer side is glued against the inner side of the label. Alternatively, cut labels are stacked, provided in magazines and are removed individually during the labelling process. Wrap-around labels are predominantly suitable for non-conical containers, or for the cylindrical regions of a container, but every container material can advantageously be labelled in such a manner, for example containers of plastics, such as PET bottles, glass, metal or cardboard.
The destacking of the cut labels is problematic with wrap-around films of thermoplastic film. The container which is already formed is labelled with the wrap-around labelling. The processing speeds are therefore considerably higher than with in-mould methods, with modern plants for example at least 10,000 containers per hour. Even with such high cycle times, it has to be ensured that the cut and stacked label can be separated well and reliably at these high speeds. Adhesion and freedom of bubbles are however not a problem with the wrap-around label.
A further difference between the wrap-around labels and the in-mould labels are the usual printing methods. During the printing of wrap-around labels, the films are initially cut to large format sheets due to cost reasons, onto which are printed several masters side by side. In this printing process, the stacked sheets are also separated with very high cycle times of at least 1000 sheets per hour. The individual wrap-around labels are subsequently cut from the printed sheets and are themselves also stacked. Due to economical reasons, it is desirable to accommodate as many printed images on the sheet as possible, that is, the larger the sheet, the lower the printing costs. But there are limits to this optimisation. The larger the sheets, the more difficult the handling of the film sheets at high cycle times, in particular, the sheets cannot be separated reliably with these destacking speeds during the entry into the printing machine.
EP0798217 describes a method for the wrap-around labelling of containers. According to these teachings, the labels are crimped immediately prior to the application, so as to reduce tensions on the adhesion seam. The specification mentions wrap-around labels made of thermoplastic film.
DE 3515324 describes labelling machines for applying wrap-around labels. This specification does not describe the labelling materials in detail.
EP 0 611 102 describes films of thermoplastic polymers which are used as in-mould labels. The films are constructed of a vacuole-containing base layer and an intermediate layer of propylene homopolymer and covering layers on both sides. The outer covering layer is printable and glossy and constructed of propylene mixture polymers. The opposite covering layer is applied to the base layer and contains two incompatible polymers and is matt. It is described that the matt inner side reduces the blocking against the printable side, whereby individual labels can be removed more easily from the stack. This film is nevertheless still in need of improvement regarding its processability as wrap-around label. In particular, large format sheet cuts cannot be reliably destacked from this film.
In the prior art, a large number of films with a matt or rough surface are known. As mentioned above, the use or polymer mixtures from incompatible polymers in covering layers is often described. This technique is used with transparent and opaque packaging films and with in-mould labels. With the packaging films, a flawless, even, matt appearance is primarily important, whereas the matt layer is supposed to avoid air bubbles between the injected container and the label with in-mould labels. In addition to the mixture of incompatible polymers, further techniques for the generation of rough surfaces are known.
In the prior art, the roughing of the surface by mechanical stamping, for example, is also described. This structuring is particularly advantageous with in-mould labels, so as to enable the escape of the air enclosed between the container and the label.
β nucleating agents, which are added to a propylene homopolymer layer, cause similar effects, whereby a microporous layer is generated. This microporous layer also enables a good deaeration between the container and in-mould label.
Coating films with a filled coating, for example kaolin, is described, whereby a rough surface is produced. Alternatively, high concentrations of pigments can be incorporated into the coextruded covering layers to generate surface roughness.
However, all known label films have the disadvantage that the destacking of the cut and the stacked labels is in need of improvement, particularly with large cuts and high cycle times, the known films cannot be separated reliably.